The invention relates to the measurement of relatively low temperatures, for example between 30.degree. C. and 500.degree. C., and more particularly the measurement of temperatures in areas which are difficult to reach or dangerous and where the environment is disrupted (lack of visibility, ambient temperature, local pressure, vibrations, existence of electro-magnetic fields, presence of ionizing radiation . . . ), for targets such as elements or objects out of reach, that is which are moving, or under high electric tension.
The invention relates more particularly to an optical pyrometer with optical fibres.
At present, it is normal to use, for temperature measurement, thermocouples or contact probes, optical pyrometers, and silica optical-fibre pyrometers.
All these solutions present inconveniences.
Thermocouples and contact probes require intimate physical thermal contact with the target of which it is desired to measure the temperature. Also, these types of thermocouples and probes are sensitive to electro-magnetic parasites and their response time is relatively long.
Optical pyrometers require direct vision of the target of which it is desired to measure the temperature and also that the optical path followed by the flux is transparent in the infra-red wavelength band where the main emission for the range of temperatures concerned is situated. It is therefore necessary to use special optical windows in the enclosures or intermediate walls.
Silica optical-fibre pyrometers are themselves not sensitive to temperatures lower than about 500.degree. C. This is due to the fact that fibres made with this substance have a transmission coefficient which is very inadequate in the infra-red.